A neat route they could've gone is to, instead of having such a large room, have something like Quark's holosuites, but small enough where only one person could go in at a time... maybe 10 ft by 10 ft or so. Have a row of these, and each person steps into their own chamber, and are joined together in one simulation, each person's environment simulated individually. I guess people couldn't touch each other, though... unless the touching itself is simulated!
If they can see eachother, they can touch eachother. If tables and AI are solid, other users should be too. Its a shame they never explored networked holodecks. Time delayed communications were only an issue on the occasion that the writers thought it might add tension, so real time networked simulations over subspace should be easy. Sleep with your long term partner in another sector, meet mates for beer on separate worlds, or log on to the ultimate holodeck edition of World of Warcarft.
The possibilities are endless.
It's entirely possible that in "Take Me Out to the Holosuite" we're seeing a complex sitautaion where no one character was interacting with a "real" counter-character. Given the size of a baseball field that alone means that in order for each character be in their positions on the field that each character is sectioned off in their own "mini holodeck" where they're only running in place and such, further only interacting with holosimulations of what the other chracters are doing in their own sections of the holodeck. It's also possible the DS9 and Vulcan teams were in seperate holosuites. We, the viewer, see everything spliced and but together as though eveything was happening "fer reals."
Now, the holodeck itself obviously has a limited level of "AI" where it can pretty much listen in to what is going on and "know" how to make objects behave.
First of all, I'd say any object a character isn't physicaly holding is "real" therefore anything they're looking at is projected. A massless image. Picard picks up a book and tosses it out the door. When he hold onto the book the book is "replicated" it is "real." Picard tosses the book out the door. Once the book is out of his hands it is no longer "real" and is now an image that will not exsist outside the holodeck. (The holodeck may also have "known" Picard's demonstration and acted accordingly.)
Now, Picard gets hit by a snowball. The holodeck "knows" that the fight is occuring and that the balls should probably remain replicated to make the fight that much more "real." So Wesley tries to avoide the snowball by calling for the exit and running into the hall. Well, sadly for him (and moreso Picard) the holodeck has a way around this.

Geordi carries the E-D sketch out of the holodeck: Simple. It's replicated since Geordi has to physically hold it. It makes sense that simple objects like that would be replicated and go outside the holodeck if someone takes it with them.
DS9: Nog's recovery with Vic in the holosuite. This is a nice one. Nog has obviously changed clothes during his stay in there so when Vic turns it off Nog isn't naked because his suit is on him and, thus, "replicated." But there's also a really nice touch in this episode. In this episode Nog enters the holosuite with a cane (as he's recovering from being injured on duty and being outfited with an artificial limb) when Vic turns off the holosuite Nog doesn't have his cane with him, infact it's nowhere nearby. But when Vic turns off the holosuite the cane drops down as it is a physical object that was brought in by Nog. (Presumably Nog's original clothes were discarded by the holodeck as is probably standard procedure for some articles of laundry and/or to maintain a 'matter balance' -that is the matter that makes up Nog's 60s suit is having its spent energy repalced by the clothes Nog walked in with. It was a nice little touch in that episode to have the cane appear like that.